Production of highly knotted DNA by means of cosmid circularization inside phage capsids

which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Background: The formation of DNA knots is common during biological transactions. Yet, functional implications of knotted DNA are not fully understood. Moreover, potential applications of DNA molecules condensed by means of knotting remain to be explored. A convenient method to produce abundant highly knotted DNA would be highly valuable for these studies. Results: We had previously shown that circularization of the 11.2 kb linear DNA of phage P4 inside its viral capsid generates complex knots by the effect of confinement. We demonstrate here that this mechanism is not restricted to the viral genome. We constructed DNA cosmids as small as 5 kb and introduced them inside P4 capsids. Such cosmids were then recovered as a complex mixture of highly knotted DNA circles. Over 250 µg of knotted cosmid were typically obtained from 1 liter of bacterial culture. Conclusion: With this biological system, DNA molecules of varying length and sequence can be shaped into very complex and heterogeneous knotted forms. These molecules can be produced in preparative amounts suitable for systematic studies and applications

La conceptualización de la variable en la enseñanza media

Se ha mostrado que aun después de cursar algebra durante varios años, los estudiantes universitarios tienen dificultades serias para comprender los usos elementales de la variable. En este trabajo se presentan y analizan los resultados de un estudio en el que se investigó la comprensión del concepto de variable en los diferentes grados de la enseñanza media. El estudio se realizó con 98 estudiantes de entre 12 y 19 años. Los resultados muestran que las concepciones de la variable que tienen los estudiantes en los diferentes cursos, no reflejan una diferencia sustancial en la comprensión de este concepto. Consideramos que las dificultades que manifiestan los estudiantes están fuertemente influidas por las practicas docentes y el contenido de los cursos de algebra

Diseño de un cuestionario de diagnóstico acerca del manejo del concepto de variable en el álgebra

This paper reports the first stage of a project investigating college novice students' interpretations and uses of different realisations of the concept of variable, namely: as unknown, as general number, in functional relationships. In particular, we present and discuss the reliability of a questionnaire that aimed at providing a first students' profile concerning their interpretations and uses of variable. Finally, initial results obtained by analysing students' responses are outlined

¿Mejora la comprensión del concepto de variable cuando los estudiantes cursan matemáticas avanzadas?

Algunas investigaciones sugieren que los estudiantes han alcanzado un pensamiento algebraico maduro cuando son capaces de usar la variable de manera flexible, esto es, cuando logran integrar sus diferentes usos y diferenciarlos. Sin embargo, se ha demostrado que el concepto de variable es difícil para los estudiantes de distintas edades, y que en los diferentes niveles educativos, los estudiantes tienen dificultades para comprender los varios usos y aspectos que caracterizan a la variable. Esta investigación se propone analizar si los estudiantes alcanzan esta comprensión conforme progresan en el estudio de las matemáticas universitarias. Para ello, se analiza cuáles aspectos de la variable utilizan estudiantes de diferentes niveles educativos (3º de secundaria, estudiantes de recién ingreso a la universidad, estudiantes de 5º semestre de las carreras de Economía e Ingeniería) al resolver problemas algebraicos. Los resultados muestran que, si bien los estudiantes usan con mayor flexibilidad estos aspectos conforme progresan en los cursos de matemáticas avanzadas, su pensamiento algebraico no se desarrolla como se esperaría

Three strategies to stabilise nearly monodispersed silver nanoparticles in aqueous solution

Silver nanoparticles are extensively used due to their chemical and physical properties and promising applications in areas such as medicine and electronics. Controlled synthesis of silver nanoparticles remains a major challenge due to the difficulty in producing long-term stable particles of the same size and shape in aqueous solution. To address this problem, we examine three strategies to stabilise aqueous solutions of 15 nm citrate-reduced silver nanoparticles using organic polymeric capping, bimetallic core-shell and bimetallic alloying. Our results show that these strategies drastically improve nanoparticle stability by distinct mechanisms. Additionally, we report a new role of polymer functionalisation in preventing further uncontrolled nanoparticle growth. For bimetallic nanoparticles, we attribute the presence of a higher valence metal on the surface of the nanoparticle as one of the key factors for improving their long-term stability. Stable silver-based nanoparticles, free of organic solvents, will have great potential for accelerating further environmental and nanotoxicity studies

mwr Xer site-specific recombination is hypersensitive to DNA supercoiling

The multiresistance plasmid pJHCMW1, first identified in a Klebsiella pneumoniae strain isolated from a neonate with meningitis, includes a Xer recombination site, mwr, with unique characteristics. Efficiency of resolution of mwr-containing plasmid dimers is strongly dependent on the osmotic pressure of the growth medium. An increase in supercoiling density of plasmid DNA was observed as the osmotic pressure of the growth culture decreased. Reporter plasmids containing directly repeated mwr, or the related cer sites were used to test if DNA topological changes were correlated with significant changes in efficiency of Xer recombination. Quantification of Holliday junctions showed that while recombination at cer was efficient at all levels of negative supercoiling, recombination at mwr became markedly less efficient as the level of supercoiling was reduced. These results support a model in which modifications at the level of supercoiling density caused by changes in the osmotic pressure of the culture medium affects resolution of mwr-containing plasmid dimers, a property that separates mwr from other Xer recombination target sites

Sliding window averaging in normal and pathological motor unit action potential trains

Objective: To evaluate the performance of a recently proposed motor unit action potential (MUAP) averaging method based on a sliding window, and compare it with relevant published methods in normal and pathological muscles. Methods: Three versions of the method (with different window lengths) were compared to three relevant published methods in terms of signal analysis-based merit figures and MUAP waveform parameters used in the clinical practice. 218 MUAP trains recorded from normal, myopathic, subacute neurogenic and chronic neurogenic muscles were analysed. Percentage scores of the cases in which the methods obtained the best performance or a performance not significantly worse than the best were computed. Results: For signal processing figures of merit, the three versions of the new method performed better (with scores of 100, 86.6 and 66.7%) than the other three methods (66.7, 25 and 0%, respectively). In terms of MUAP waveform parameters, the new method also performed better (100, 95.8 and 91.7%) than the other methods (83.3, 37.5 and 25%). Conclusions: For the types of normal and pathological muscle studied, the sliding window approach extracted more accurate and reliable MUAP curves than other existing methods. Significance: The new method can be of service in quantitative EMG